This application relates to a method for forming superconducting materials through the use of chemical vapor deposition and more specifically, it involves a preparation of superconducting films through the use of metal organic chemical vapor deposition of volatile metal organic complexes.
The recent discovery of metal oxide ceramic materials with relatively high superconducting transition temperatures gives rise to a large number potential applications in advanced materials based technologies. However, appropriate practical synthesis and processing methodologies for forming the superconductive ceramics are required for the use of superconducting materials in microelectronics and optoelectronics. Particularly desirable superconductors are [TlO.sub.m Ba.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.2n+2 ; m=1,2; n=1 ,2,3] and [(BiO).sub.2 Sr.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.x n=1,2,3]. In general, such superconductive ceramics may be formed as high-quality thin films which are deposited from the gas phase. These thin films have in the past been deposited by physical vapor deposition techniques (ion-sputtering, electron beam, plasma, laser) which employ metal and metal oxide sources. All of these vapor deposition techniques require expensive and/or energy-intensive equipment, must be carried out at high temperatures and do not provide consistently good quality films.
Metal organic chemical vapor deposition has been used for depositing thin films from a vaporizable material. When using metal organic chemical vapor deposition (MOCVD) for depositing metal oxide films on substrates, volatile metal organic precursors must be used. Metal organic precursors for the preparation of the known superconductive ceramic materials which are stable in the gas phase are not generally available. However, the chemical vapor deposition technique does potentially offer the advantage, when compared to the physical methods of film preparation, of higher quality, smoother and more crystalline films produced at lower temperatures and amenability to large scale deposition.
In order to use metal organic chemical vapor deposition, the metal precursor compounds must be sufficiently volatile to transport the compound as a gas to the substrate site. In addition, the metal precursor compound must be relatively stable in the gaseous state. Coordination number and coordinative saturation appear to play a major role in determining the Volatility of neutrally charged metal organics. In general, in low oxidation states, a metal complex with non-bulky ligands will attempt to expand its coordination number by coordinating additional ligands, or, when these are unavailable, by forming ligand bridging bonds to neighboring ions. Such bridging bonds lead to polymeric structures, high lattice energies, and low volatility. Two approaches can be used to circumvent this problem. In higher oxidation states, the metal will have a smaller ionic radius, hence a smaller desired coordination number, and generally more non-bridging ligands per metal ion. This usually leads to higher volatility. Another way to reduce intermolecular interactions is by employing sterically bulky and/or fluorinated ligands. Also important, but apparently less critical, is keeping the molecular weight as low as possible.
The compounds selected for use in metal organic chemical vapor deposition must be stable to the thermal conditions necessary to volatilize them. In general, many of the properties which impart volatility additionally afford some resistance to thermal degradation, and therefore, the approaches which should foster higher volatility are also reasonable initial approaches to enhance thermal stability.
As a result, the optimum manner for rendering metal complexes volatile should be complexation by bulky and/or multidentate non-polar hydrocarbon ligands, which saturate the coordination sphere and prevent oligomerization. Ligand fluorination further enhances volatility.
Among the currently known classes of high T.sub.c superconductors, Tl-Ba-Ca-Cu-O materials [TlO.sub.m Ba.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.2n+2 ; m=1,2; n=1,2,3] and Bi-Sr-Ca-Cu-O materials [(BiO).sub.2 Sr.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.x n=1,2,3] exhibit a multiplicity of superconducting phases, high environmental stability, ease of synthesis, and T.sub.c values as high as 122K. Such valuable characteristics have stimulated efforts to prepare high quality Tl-Ba-Ca-Cu-O and Bi-Sr-Ca-Cu-O thin films, and to date activity has largely centered on physical vapor deposition techniques such as sputtering, multi-layer thermal evaporation, and electron beam evaporation. If appropriate molecular precursors and deposition chemistry could be developed, metal organic chemical vapor deposition (MOCVD) would offer an attractive alternative approach and the potential advantages of simplified deposition apparatus, high deposition rates, efficiency in coating complex shapes, excellent film uniformity, and adaptability to deposition under highly oxidizing conditions.